Various types of regulation golf balls have been proposed. In order to meet the United States Golf Association ("U.S.G.A.") specifications, the golf ball must be spherical in shape, have equal aerodynamic properties and equal moments of inertia about any axis through its center. The ball must have a minimum diameter of 1.68 inches (4.267 centimeters), a maximum weight of 1.620 ounces (45.926 grams), and a maximum initial ball velocity of 255 feet per second as measured on a standard U.S.G.A. ball testing machine.
Golf balls are generally of two kinds, a wound golf ball and a molded golf ball. Because molded golf balls are cheaper to produce and more durable than conventional wound golf balls, efforts have been focused on designing molded balls to improve such characteristics as initial velocity, number of revolutions, angle of departure, moment of inertia, and dimple form. Most golf balls presently manufactured are two-piece balls of uniform density cores throughout.
More recent golf ball designs have, however, focused on solid, multi-piece golf balls wherein the density of the ball is not uniform throughout. For example, U.S. Pat. No. 4,863,167 to Matsuki et al. discloses a three-piece molded golf ball which includes a center portion, an outer layer disposed over the center portion, and a cover disposed over the outer layer. The center and outer layers are formed from a rubber composition containing a base rubber. The rubber composition of the outer layer contains a gravity filler with a high specific gravity such that the outer layer has a higher specific gravity than the center of the solid core. The gravity filler may be selected from tungsten, tungsten carbide, molybdenum, lead, lead dioxide, nickel, or copper.
Other golf ball designs consider both the weight or density distribution of the ball and the relative hardness of the various layers which comprise the golf ball. For example, U.S. Pat. No. 4,979,746 issued to Gentiluomo discloses a two or three piece molded type golf ball having an elastic center. The elastic center has a minimum compressibility of at least ten percent greater than the contacting synthetic elastomer composition which is highly resilient and has a minimum Shore A durometer hardness of about 70. The softer elastic center such as plastic elastomer or rubber, plastic or rubber foam, natural or composition cork, allows each ball to flatten more under club impact, to reduce likelihood of ball breakage and provide for excellent click and feel. When the center is made of low density material, more weight is allowed to be concentrated within the outer portion of the ball to provide a ball exhibiting reduced hooking and slicing action when improperly hit.
U.S. Pat. No. 5,026,067 also issued to Gentiluomo discloses a regulation golf ball comprising a low density center having a maximum diameter of 1.25 inches, a molded encapsulating mass surrounding the center, wherein the material in contact with the center is a highly resilient synthetic elastomer composition having a minimum Shore A durometer hardness of 70 and a specific gravity of 1.0, and a patterned surface contouring of predetermined structure contained within the outer surface of the golf ball. The center has a compressibility of less than ten percent of the compressibility of the material in contact therewith, and a density less than the maximum prevailing density of the encapsulating mass.
U.S. Pat. No. 5,273,286 issued to Sun discloses a multiple concentric section golf ball comprising four sections each having a spherical outer surface, and all sections having a common center. The first section is an inner core closest to the center and consists of substantially incompressible material. The second section is an intermediate core consisting essentially of carbonaceous material, and the third section is an outer core in the form of a shell surrounding the intermediate core which consists essentially of an elastomer. The fourth section is a cover in the form of a shell surrounding the outer core. The radius of gyration and spin rate of the golf ball can be controlled by selection of the weight, density, and size of each of the first, second and third sections.
More recently, U.S. Pat. No. 5,480,155 issued to Molitor et al. discloses a golf ball comprising a hollow, spherical shell of a polymeric material; a unitary, noncellular core of a material which, at the time of introduction into the shell, is a liquid, and a one-piece spherical cover over the center. According to the specification, the spherical shell, as opposed to the core, is primarily responsible for the high initial velocity obtained when the golf ball is struck by a golf club so as to allow the golf ball to be driven long distances. The preferred shell compositions are ionomers. Preferably, the core material has a specific gravity greater than that of the shell.
Except for the ball disclosed in Molitor, the above designs provide for a golf ball wherein the solid core of the golf ball becomes heavier closer to the cover of the ball to improve the moment of inertia thereby improving the spin and flight characteristics. While others have attempted to provide golf ball designs so that the moment of inertia and the spin rate can be controlled by selection of the weight or density and size of the materials comprising the golf ball, these designs are limited in the degree to which the higher density can be concentrated close to the cover or outer edge of the golf ball. Additionally, the use of fillers such as zinc oxide, barium sulfate, calcium carbonate, and other non-polymer compounds to increase the weight distribution of the ball toward the outer core adversely affect other important performance characteristics such as rebound.
Therefore, a golf ball design having a much improved moment of inertia over current golf ball designs which allows the ball to possess improved performance characteristics of low spin for maximum distance while maintaining optimum rebound characteristics is desired.